3.1.10 Reducing the Clock Frequency

Now let’s look at how we can reduce the power consumption of the application, and verify that it is improved.

A first thought might be to get rid of the delay loop and run the LED blinker of a timer interrupt. In addition, a simple example like this does not need to run at high-speed, so we can use the clock prescaler to run slower. The code below is included in project low_power_102.

#include <avr/io.h>
#include <avr/interrupt.h>

// Timer 0 ISR
ISR (TIMER0_OVF_vect) 
{
    if (PORTB == 0x00) {
        // LED is OFF, turn it on 
        PORTB = (1 << 5);
        // Shortened timeout for on cycle
        TCNT0 = 0x102;
    }
    else {
        // LED is ON, turn it off
        PORTB = 0x00;
    }
}

int main(void)
{
    // Change the clock prescaler
    CLKPR = (1 << CLKPCE);
    // Scale by DIV64
    CLKPR = (1 << CLKPS2) | (1 << CLKPS1) | (0 << CLKPS0);
    // Port B5 to output
    DDRB = (1 << 5);
    // Timer0 DIV 1024 
    TCCR0B = (1 << CS02) | (1 << CS00); 
    // Overflow interrupt enable
    TIMSK0 = (1 << TOIE0);  
    // Interrupts on
    sei();
    // Do nothing
    while (1)
        ;
}

Leaving the application to run for a few seconds should give you a plot similar to this one: